1. Technical Field of the Invention
The present invention relates to an apparatus for improving linearity of small signal, more specifically relates to an apparatus for improving linearity of small signal which is used for transmitter and receiver.
2. Description of the Prior Art
FIGS. 1 to 3 show linearizers according to prior arts
FIG. 1 shows a block diagram illustrating a linearizer using a general predistortion method.
Referring FIG. 1, the linearizer includes a divider 110, a third-order distortion generator 120, an amplitude & phase controller 130 and a coupler 140.
The divider 110 divides an input signal from outside to two signals. One divided signal is directly transferred to the coupler 140. And the other is transferred to the coupler 140 via the third-order distortion generator and the amplitude & phase controller 130. The third-order distortion generator generates the third-order distortion signal. The amplitude & phase controller 130 receives the signal from the third-order distortion generator 120 and controls amplitude and phase of the third-order distortion signal. The coupler 140 combines the input signal from the divider 110 and the third-order distortion signal from the amplitude & phase controller 130, such that the third-order distortion of the input signal is cancelled.
FIG. 2 shows a block diagram illustrating a linearizer using a general feed-forward cancellation method.
Referring to FIG. 2, the linearizer includes a divider 210, a first amplifier 220, a directional coupler 230, a subtracter 240, a second amplifier 250 and an adder 260.
The divider 210 divides an input signal from outside to two signals. One divided signal is transferred to the first amplifier 220 and the other is transferred to the subtracter 240. When the signal is amplified in the first amplifier 220, the signal is distorted. The output signal from the first amplifier 220 including distortion is transferred to the subtracter 240 and the adder 260 via the directional coupler 230.
When the subtracter 240 subtracts the amplified and distorted signal, outputted from the first amplifier 220, from the output signal of the divider 210, the signal from the divider 210 and amplification of the output signal from the first amplifier 220 are cancelled and distortion of the output signal from the first amplifier 220 is reversed. The reversed and distorted signal is amplified by the second amplifier and transferred to the adder 260.
The adder 260 adds distorted and amplified signal from the first amplifier 220 to the amplified and reversed distortion signal from the second amplifier 250. At this time, distortion of the output signal from the first amplifier 220 and the reversed distortion signal from the second amplifier 250 are cancelled, such that only signal amplified by the first amplifier is outputted to outside.
For example, we suppose that signal "all" is transferred to the first amplifier 220 and the subtracter 240, the first amplifier 220 outputs the amplified signal "a" and the distortion signal "b". The subtracter 240 subtracts the amplified signal "all" and the distortion signal "b" from the signal "a", thereby cancelling the signal "all" and the amplified signal "all" and outputting the reversed signal "-b". The adder 260 adds the amplified signal from the first amplifier 220 and the distortion signal "b" to the reversed distortion signal "-b" from the subtracter 240, thereby cancelling the distortion signal "b" and the reversed distortion signal "-b" and outputting only the amplified signal "a" to outside.
FIG. 3 shows a block diagram illustrating a linearizer using a general Cartesian-feedback method.
Referring to FIG. 3, the linearizer includes a first amplifier 310, a directional coupler 320, an amplitude & phase controller 330, a filter 340 and a second amplifier 350.
The first amplifier 310 amplifies the input signal from outside and outputs the amplified signal to the directional coupler 320. At this time, the amplified signal includes distortion signal which is created during amplification.
The amplified signal and the distortion signal from the first amplifier 310 is transferred to the second amplifier 350 and the amplitude & phase controller 330 through the directional coupler 320. The amplitude & phase controller 330 controls amplitude and phase of the amplified signal and reverse and transfers the distortion signal to the filter 340.
The filter 340 filters the amplified signal and the reversed distortion signal and transmits the filtered signal to the first amplifier 310. Then, the first amplifier 310 amplifies the amplified signal from the filter 340. The amplified signal is outputted to outside through the directional coupler 320. At this time, the distortion signal created in the first amplifier 310 is offset and cancelled by the reversed distortion signal from the filter 340. In this case, the distortion signal is cancelled by using the reversed distortion signal created in feedback loop.
The linearizers mentioned above may be used for a base station, but cannot be used for a small, low power and high effect terminal because of complexity and large scale of the circuit.